On the Manage Sponsor Accounts page, the Site Sponsor can remove existing user access by first selecting the user account that needs to be edited then clicking on the Remove Access icon in the Ribbon (assuming the Site Sponsor has been granted this capability):

After clicking on the Remove Access icon, the Site Sponsor will be given the option to Remove All Access from the user account, or to Remove Optional Access from the user account. Removing Optional Access will only be relevant if the Site Sponsor has been given an Optional Associative Security Definition by the SharePoint Administrator.
After selecting from the available options click Save to confirm the access removal:

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